The Northern Lake remained oligotrophic until around 1955, though the eutrophication had already started in pre-war days as seen in the past trend of transparency in Fig. ASI-1-4. However, it was suddenly accelerated by the post-war industrialization of the lake's catchment area. The first clogging trouble in the sand filter of a city water supply to Kyoto took place as early as in 1959. Between 1960 and 1965, drastic changes in the biomass and species composition of plankters and benthic animals became apparent. The plankton biomass increased almost tenfold since 1950 (Fig. ASI-1-11), while the primary productivity in Northern Lake nearly doubled between 1965 and 1985. Algal blooms, particularly the so-called "freshwater red tide" caused by Uroglena americana, and the resultant unpleasant smell of tap water from the lake became a matter of keen social concern.

The Water Pollution Control Law legislated in 1970 abated the rate of eutrophication to a considerable extent through the regulation of nutrient level in industrial effluents, but the deterioration of lake water quality did not stop at all due to the steady growth of population and industrial activity in the catchment. The construction of an extensive sewerage network started in 1972 within the framework of the Lake Biwa Comprehensive Development Project, though its progress has been slow owing to the financial burden to local communities.

The residents' voluntary movement against the use of phosphate-containing synthetic detergents resulted in the ban of their use in 1980 by the enforcement of a prefectural ordinance for the prevention of eutrophication of L. Biwa. The P content of lake water was thereby somewhat reduced in past several years, but the effect of the ordinance has been only marginal. To prevent further eutrophication, it seems urgent to take new measures at least until the completion of the sewerage network.